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/**
 * Add the field to the checkout
 */
add_action( 'woocommerce_after_order_notes', 'my_custom_checkout_field' );
 
function my_custom_checkout_field( $checkout ) {
 
    echo '<div id="my_custom_checkout_field"><h2>' . __('Codice Fiscale / Partita Iva') . '</h2>';
 
    woocommerce_form_field( 'p_iva', array(
        'type'          => 'text',
        'class'         => array('my-field-class form-row-wide'),
        'label'         => __('Partita Iva o Codice Fiscale *'),
        'placeholder'   => __('Partita Iva o Codice Fiscale'),
        ), $checkout->get_value( 'my_field_name' ));
 
    echo '</div>';
 
}

/**
 * Process the checkout
 */
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');
 
function my_custom_checkout_field_process() {
    // Check if set, if its not set add an error.
    if ( ! $_POST['p_iva'] )
        wc_add_notice( 'P.IVA ' . __( 'is a required field.', 'woocommerce' ), 'error' );
}
/**
 * Update the order meta with field value
 */
add_action( 'woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta' );
 
function my_custom_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['p_iva'] ) ) {
        update_post_meta( $order_id, 'P.IVA', sanitize_text_field( $_POST['p_iva'] ) );
    }
}
/**
 * Display field value on the order edit page
 */
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );
 
function my_custom_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('P.IVA').':</strong> ' . get_post_meta( $order->id, 'P.IVA', true ) . '</p>';
}
/** * Add the field to order emails **/
add_filter('woocommerce_email_order_meta_keys', 'my_custom_checkout_field_order_meta_keys'); 
function my_custom_checkout_field_order_meta_keys( $keys ) {	
	$keys[] = 'P.IVA';	 
	return $keys;
}